Corrected entry: Malagant imprisons Guinevere in an 'oubliette', which is French for 'place of forgetting', a dungeon where prisoners are left to rot. But oubliettes were only in use during the Middle Ages, centuries after King Arthur's time, and even then they were not platforms over pits as the film depicts. They were dungeon cells beneath the castle floors.
Corrected entry: Towards the end, in the Great Square, Maligant sends his men in to burn the city. Immediately, the Knights of the Round Table start to defend their city. One of the Knights is shot by an arrow, and is presumably dead. However, at the end when King Arthur is floating in the lake, the knight is fine.
Corrected entry: The stories of King Arthur take place in the Dark Ages, between 500-1000 AD. In the movie, lots and lots of people, mostly Arthur and his knights, wear blue clothes. Yet blue dye wasn't used in Europe until Marco Polo brought it from China in 1274, and even then it was extremely expensive.
Corrected entry: Right at the end, when a burning arrow is shot at the "floating tomb" in order to set it on fire, flames appear on it before the arrow hits it.
Corrected entry: One of Malagant's knights holds Guinevere captive and tells Lancelot to throw down his sword. Lancelot drops the weapon at his own feet and takes a few steps forward to reason with the knight. Later on when the knight is dead, Lancelot simply bends down and picks up his sword from the ground. How did it get there? By right it should have been a few yards BEHIND him.